Optical instruments such as binoculars and telescopes which include an objective lens and an ocular lens have been known. When both the objective lens and the ocular lens are constructed so as to have positive refractive power, an inverted image is formed. In astronomical telescopes this is acceptable. However, in binoculars and terrestrial telescopes, an erecting optical system is placed between the objective lens and the ocular lens in order that the observer can observe an erect image. For example, the erecting optical system often includes an erecting prism, as used in so-called prism binoculars. More particularly, it is common to use a so-called Schmidt erecting prism in the erecting optical system when making compact binoculars.
FIGS. 7(A) and 7(B) show an optical system of a prior art optical instrument for observation that uses a Schmidt prism to form an erect image. This optical system is provided with a positive objective lens 110, an erecting optical system 130 and a positive ocular lens 120. Moreover, the position xe2x80x9c111xe2x80x9d on the optical axis Z1 represents the axial position of an image that is formed by the objective lens 110, and xe2x80x9cE. P.xe2x80x9d represents the pupil position for observation (eye point).
As shown in FIGS. 7(A) and 7(B), the erecting optical system 130, termed a Schmidt prism, is actually formed of two prisms, namely, a first prism 131 and a second prism 132 which are almost touching but are separated by a small air gap. The first prism 131 has three reflecting, planar surfaces that are active, namely, surfaces 131A, 131B, and 131C. The second prism 132 is a roof prism and has a roof surface 132C formed of two reflection surfaces that are perpendicular to each other. The second prism 132 has three reflecting, planar surfaces that are active, namely, surfaces 132A, 132B and the roof surface 132C.
In an optical instrument for observation having such an optical system, light emergent from the positive objective lens 110 is first incident upon the surface 131A of the erecting prism 130, at which point it is transmitted. Then the light undergoes total internal reflection at the surface 131B and is directed to the surface 131C, where it again undergoes total internal reflection. When the light is again incident onto surface 131B, its incident angle is less than that required for total internal reflection, and so the light is transmitted. Thus, the light is transmitted through surface 131B and is incident onto the surface 132A of the second prism 132.
The light incident upon the second prism 132 undergoes total internal reflection at the surface 132B, is reflected by a mirror at surface 132C, and undergoes total internal reflection at surface 132A, and then is emergent from the surface 132B and is transmitted to the ocular lens 120. The inverted image that would normally be formed by the positive objective lens 110, by the action of the erecting prism 130, is converted to an erect image. This erect image is then observed by the ocular lens 120.
When a Schmidt erecting prism 130 is used to erect an image, there is a problem in that ghost light, as will be explained below, may be generated. Namely, as shown in FIGS. 7(B) and 8, light 140 that is incident upon the first reflection surface (i.e., the surface 131B of first prism 131) at an angle xcex81 (FIG. 8) that is smaller than the critical angle (the angle required for total internal reflection) is transmitted by the first reflection surface and is emergent from the first prism 131 without being reflected by the first prism 131. Subsequently, this light then passes through the second prism 132 and the ocular lens 120 and appears as ghost light that deteriorates the quality of the image seen by the observer.
In optical instruments for observation, such as monoculars and binoculars, if a vibration occurs so as to deviate the optical axis of an optical instrument for observation from the viewing direction, an angular deflection of the light rays occurs. Thus, the quality of an observed image may be greatly degraded, especially in the case where the image is observed with a high magnification. Various anti-vibration mechanisms have been proposed for optical instruments in order to prevent the angular deflection of light rays from an observed object due to a vibration. For example, a mechanism has been disclosed which maintains the spatial orientation of an erecting optical system, such as the so-called Schmidt prism 130, substantially constant despite a vibration or sudden change in the direction of the optical axis of the optical instrument.
However, ghost light is particularly troublesome in an optical system that has been provided with such an anti-vibration or image-stabilizing mechanism. In such an optical system, the spatial orientation of an erecting optical system, such as the Schmidt prism 130, is maintained constant during a vibration while the objective lens 110 and the ocular lens 120 are rotated by the vibration relative to the Schmidt prism 130. When the optical system rotates as a result of a vibration or sudden change in the direction of the optical axis of the optical instrument, the objective lens 110 receives light that originally was outside the effective diameter of the objective lens 110. Some of this light may not undergo total internal reflection in the Schmidt prism, such as at surface 132A, and will produce ghost light that will degrade the quality of the observed image.
The present invention relates to an optical instrument for observation which uses an inverting optical system (erecting system) for forming an erect image, and more particularly, relates to an optical instrument for observation such as binoculars or a telescope, etc., that is provided with an anti-vibration mechanism. In particular, the present invention relates to an optical instrument for observation which can prevent the occurrence of ghost light.